1. Field of the Invention
The present invention relates to an optical receiver with a current mirror circuit, in particular, the invention relates to an optical receiver with a current mirror circuit implementing with MOSFETs.
2. Description of Prior Arts
An optical receiver that receives an optical signal and converts it to a corresponding electrical signal generally provides a function to monitor an average power of the optical signal. In the optical receiver including a photodiode (hereafter denoted as PD), the circuit detects an average of the photocurrent generated in the PD by inserting a resistor in a bias line to the PD and by detecting a voltage difference in this resistor caused by the photocurrent. However, because the dynamic range of the input optical power varies over 3-digits or more, from a few micro-decibel (dBμ) to a few milli-decibel (dBm) or more, the technique above described to detect the optical power through the voltage difference by the resistor has an inherent subject of the inaccuracy in a case of lower optical powers and the insufficient bias voltage for the PD in case of higher optical powers.
Another method has been known in place of the sensing resistor described above, in which the optical receiver provides a current mirror circuit between the bias power supply and the PD. The current mirror circuit may reflect the photocurrent generated by the PD to a mirror current to be processed by the optical receiver. The current mirror circuit generally provides bipolar transistors operated in the saturation region, where the collector current becomes independent of the collector-emitter bias, to show a fine current-mirror characteristic. However, it is necessary to set the forward bias condition for the p-n junction between the base and the emitter of the bipolar transistor to operate the transistor in the saturation region. This forward bias condition reaches at least 0.6 to 0.7 V, and the PD is biased by a voltage that the forward bias voltage is subtracted from the bias power supply.
On the other hand, recent electronic equipments continuously request the reduction of the power consumption. The optical receiver is not an exception of the request. For instance, the optical receiver installing a PIN-PD had been allowed to implement with a power supply of 5V. However, recent specifications of the optical receiver request the power supply of 3.3 V or less. Under such a circumstance, the forward bias condition of about 0.7 V for the bipolar transistor becomes a weighty factor.
Because the current mirror circuit using bipolar transistors becomes apparent to have inherent subjects described above, in particular, the subject under a lower power supply, the MOSFETs instead of the bipolar transistor has been practical. However, the MOSFETs has a lower voltage gain compared with that of the bipolar transistor and are hard to get a satisfactory current mirror characteristics. The present invention is to provide a current mirror circuit implemented with MOSFETs that shows good mirror performance.